Sandbox 42



Hen Egg-white Lysozyme Overview
The primary catalytic action of the enzyme hen egg-white lysozyme is to hydrolyze β(1→4) glycosidic linkages found in bacterial cell walls. More specifically, lysozyme hydrolyzes the linkages from N-acetylmuramic acid to N-acetylglucosamine which occur in peptidoglycans of the cell wall. The small 14.3 kD Hen egg-white lysozyme is one of the most widely studied lysozymes.

Secondary structure
Hen egg white lysozyme is formed from one polypetide chain 129 amino acids in length. Important secondary structures formed by the chain include 7 alpha helices and 1 beta sheet consisting of 3 anti-parallel strands. 4 disulfide bonds are involved in folding of the chain.

Distribution of residue polarity
The distribution of hydrophobic and polar residues in lysozyme is varied, with both types of residues on the surface of the enzyme. Water interacts with polar residues of the enzyme both on the exterior and interior regions of the enzyme.

Active site and Binding
Hen white lysozyme's substrate binding site accomodates six residue oligosaccharides. Glu 35 and Asp 52 are the enzyme's active site residues. These residues have distinctly different microenvironments which are critical for their catalytic action. Asp 52 forms hydrogen bonds with surrounding residues including Asn46, Asp48, Ser50 and Asn59 on the anti-parallel beta-sheet and is negatively charged allowing for electrostatic stabilization of the reaction intermediate. Glu 35 conversely is surrounded by hydrophobic residues and its side chain stays protonated allowing for acid catalysis. Catalysis proceeds through the formation of a covalent intermediate. A mutation of T4 lysozyme allows for its product to stay bound to the enzyme. This mutation made it possible to isolate a a covalent-substrate intermediate which also shows the predicted distortio n of the sugar in the 4th position of the active site.

Comparative Structures
Hen egg-white lysozyme is a c-lysozymes in a family of lysozymes which also includes alpha-lactalbumins. Alpha-lactalbumins and c-lysozymes have very similar sequences and structures, including 4 conserved disulfide bonds However, alpha-actalbumins ligand with calcium and serve the different function of regulating the biosynthesis of milk lactose